Module canceller 

Cancel request broadcaster for redundant order cancellation.

This module provides the CancelBroadcaster which fans out cancel requests to multiple HTTP clients in parallel for redundancy. Key design patterns:

• Dependency injection via traits: Uses CancelExecutor trait to abstract the HTTP client, enabling testing without #[cfg(test)] conditional compilation.
• Trait objects over generics: Uses Arc<dyn CancelExecutor> to avoid generic type parameters on the public API (simpler Python FFI).
• Short-circuit on first success: Aborts remaining requests once any client succeeds, minimizing latency.
• Idempotent success handling: Recognizes “already cancelled” responses as successful outcomes.

Structs

BroadcasterMetrics

Broadcaster metrics snapshot.

CancelBroadcaster

Broadcasts cancel requests to multiple HTTP clients for redundancy.

CancelBroadcasterConfig

Configuration for the cancel broadcaster.

ClientStats

Per-client statistics.